Daeho Jeong

Fatigue crack propagation behavior of Fe24Mn steel weld at 298 and 110 K

The fatigue crack propagation (FCP) tests were conducted on Fe24Mn steel in the region of base metal (BM), weld metal (WM) and fusion line (FL) at 298 and 110 K. The FCP rates of Fe24Mn specimens in the region of BM, WM and FL were greatly decreased, while no notable difference in the fracture mode was observed, with decreasing temperature from 298 to 110 K. The FCP rates of Fe24Mn WM and FL specimens were slightly lower than those of BM specimen at both room and cryogenic temperatures. The SEM fractographic analyses suggested that each specimen showed the transgranular facets at both temperatures in low and intermediate ΔK regimes. However, the morphological details varied depending on the region of weld and testing temperature. Relatively large sized facets were observed for the WM specimen with the columnar grain boundary playing the same role as the grain boundary in the BM and the FL specimens in the near-threshold ΔK regime. The FCP behavior of Fe24Mn steel in the region of BM, WM and FL is discussed at 298 and 110 K based on the fractographic and micrographic observation.

Ambient and cryogenic S-N fatigue behavior of Fe15Mn steel and its weld

The S-N fatigue behavior of Fe15Mn (Fe-0.7C-15Mn-2Al) austenitic steel, including base metal and butt-welded joint, was investigated at 298 K and 110 K, and the results were compared to those of STS304L (Fe-1Si-2Mn- 20Cr-10Ni) counterparts. Both specimens showed improved resistance to S-N fatigue with decreasing temperature from 298 K to 110 K. The resistance to S-N fatigue of Fe15Mn steel was greater at 298 K, while it was lower at 110 K, than STS304L steel. Unlike STS304L, Fe15Mn steel did not show any notable transformation induced plasticity and twining-induced plasticity effect under fatigue loading at ambient and cryogenic temperatures. The S-N fatigue behavior of Fe15Mn steel was strongly dependent on tensile strength at both ambient and cryogenic temperatures. Similar S-N fatigue behavior was also observed for the butt-welded joints of Fe15Mn steel. The S-N fatigue behavior of Fe15Mn steel and its weld was discussed based on the fractographic and microscopic observations.

Stress corrosion cracking behavior of X80 steel in artificial seawater under controlled strain rate and applied potentials

The effect of applied potential on the stress corrosion cracking (SCC) behavior of X80 steel was examined in artificial seawater (ASW) at different strain rates of 1×10−4, 1×10−5 and 1×10−6/sec. The controlled potential of −650, −850, −950 and −1,050 mVSCE, respectively, was applied during strainig. It was found that X80 steel was susceptible to SCC in seawater environment under both anodic and cathodic applied potentials and the susceptibility was sensitive to strain rate. The SCC was initiated at the surface pits under an anodic applied potential of -650 mVSCE. The effect of cathodic applied potential on the SCC behavior of X80 steel in ASW was more complex, such that the combined effect of surface damage, including pits and hydrogeninduced cracking, and hydrogen concentration generated on the surface tended to determine the SCC susceptibility. The SCC behavior of X80 steel with different applied potentials in ASW was discussed based on the microstructural and the fractographic observations.

Effect of applied potential on fatigue crack propagation behavior of Fe24Mn steel in seawater

In the present study, the fatigue crack propagation (FCP) tests were conducted on X80 steel in air and artificial seawater (ASW) under various applied potentials to establish optimum and safe working limits of cathodic protection (CP). The slow strain rate test (SSRT) was also conducted on the X80 BM specimens in ASW under CP potential to identify the susceptibility of hydrogen affecting the FCP behavior. The CP potential of −850 and −1,050 mVSCE suppressed the environmental effect of seawater on the FCP behavior of X80 BM and WM specimens, showing almost identical da/dN-ΔK curves for both air and ASW environments. The SSRT in ASW under CP potential of −1,050 mVSCE suggested that the X80 BM specimen steel is susceptible to hydrogen embrittlement, but the effect of hydrogen was believed to be marginal in affecting the FCP behavior of the X80 specimens at a loading frequency of 10 Hz. The FCP behavior of high strength X80 steel is discussed based on the fractographic observation to understand the FCP mechanism in seawater under various CP potentials.

Effect of superplastic forming exposure on tensile and S-N fatigue behavior of Ti64 alloy

The effect of superplastic forming (SPF) on tensile and S (stress)-N (number of cycles to failure) fatigue properties of Ti64 alloy was examined at 298 and 473 K. For simulating the superplastic forming exposure, millannealed Ti64 alloy sheet was heated in a vacuum chamber with a pre-determined temperature profile. For some as-exposed specimens, the α-case formed on the surface during expousre was mechanically removed to understand the effect of α-case on the mechanical properties of Ti64 alloy. It was found that the presence of α-case significantly affected the tensile and the fatigue properties of Ti64 alloy at 298 and 473 K by providing an easy initiation site for both tensile and fatigue fracture. The microstructural change during the SPF exposure was marginal in affecting the S-N fatigue properties of Ti64 alloy. Different testing temperature of 298 and 473 K affected the S-N fatigue behavior of as-received and as-exposed (α-case removed) Ti64 specimens, but not that of as-exposed specimen.

S-N fatigue behavior of Fe25Mn steel and its weld at 298 and 110 K

The S-N fatigue behavior of newly developed Fe25Mn steel, including base metal and butt-welded joint, was investigated at 298 and 110 K, and the results were compared to those of previously reported Fe16Mn2Al and STS304L steels. Fe25Mn steel has quite promising fatigue performance at 298 K and even at 110 K, showing comparable resistance to fatigue to STS304L. The S-N fatigue behavior of Fe25Mn steel was dependent on tensile strength at 298 and 110 K, the trend of which well agreed to that of other austenitic steels. The electron backscatter diffraction and micrographic analyses suggested that transformation induced plasticity and twinning induced plasticity effects did not occur in Fe25Mn steel under fatigue loading at room and cryogenic temperatures. The butt-welded Fe25Mn/Fe25Mn and Fe25Mn/STS304L specimens also showed a satisfactory fatigue behavior which was even comparable to that of STS304L/STS304L specimen at 110 K. The S-N fatigue behavior of Fe25Mn steel and its welds was discussed based on the fractographic and microscopic observations.

Effects of cooling rate and stabilization annealing on fatigue behavior of β-processed Ti-6Al-4V alloys

The effects of stabilization annealing and cooling rate on high cycle fatigue (HCF) and fatigue crack propagation (FCP) behaviors of β-processed Ti64 alloys were examined. After β-process heating above β transus, two different cooling rates of air cooling (β-annealing) and water quenching (β-quenching) were utilized. Selected specimens were then underwent stabilization annealing. The tensile tests, HCF and FCP tests on conducted on the β-processed Ti64 specimens with and without stabilization annealing. No notable microstructural and mechanical changes with stabilization annealing was observed for the β-annealed Ti64 alloys. However, significant effect of stabilization annealing was found on the FCP behavior of β-quenched Ti64 alloys, which appeared to be related to the built-up of residual stress after quenching. The mechanical behavior of β-processed Ti64 alloys with and with stabilization annealing was discussed based on the micrographic examination, including crack growth path and crack nucleation site, and fractographic analysis.

Curing viruses in Pleurotus ostreatus by growth on a limited nutrient medium containing cAMP and rifamycin

Oyster mushroom spherical virus (OMSV) and oyster mushroom isometric virus (OMIV) are the causative agents of a fruiting body deformation disease in the edible mushroom Pleurotus ostreatus. The curing of these mycoviruses was facilitated by a serial transfer of infected mycelia onto a limited nutrient medium containing 1mM of cAMP and 75 μg/ml of rifamycin (cAMP-rifamycin plate). The mycelia were grown on cAMP-rifamycin plates for 5 successive passages. ELISA and RT-PCR showed that the amount of mycoviruses inside the mycelia decreased significantly with increasing numbers of passages. The mycelia became free of viruses after 5 successive passages. Cultivation of the virus-cured mycelia on a mushroom compost medium produced a normal harvest, whereas the spawn infected with viruses failed to produce any fruiting bodies.

Fatigue crack propagation behavior of Fe25Mn and Fe16Mn2Al steels at room and cryogenic temperatures

The fatigue crack propagation (FCP) behavior of Fe25Mn and Fe16Mn2Al austenitic steels was investigated at 298 and 110 K, and the results were compared with the reported results of Fe24Mn2Cr steel. It was found that the FCP behavior of high-Mn, austenitic steels was largely influenced by the stacking fault energy (SFE) and the grain size. The resistance to FCP of high-Mn steels in this study was enhanced in the near-threshold ΔK regime with decreasing temperature from 298 to 110 K. The improvement for the Fe25Mn and the Fe16Mn2Al specimen was, however, marginal as compared to that of the Fe24Mn2Cr specimen. Other than the change in SFE, the secondary cracking at cryogenic temperature appeared to affect the FCP behavior of high-Mn steels, since the secondary cracks perpendicular to the crack propagating direction could reduce the effective stress intensity factor, decreasing the FCP rates. Sufficiently high stress concentration at grain boundary tended to occur at low temperature for relatively large grain sized Fe24Mn2Cr specimen and cause the secondary cracking, but not for the Fe25Mn and the Fe16Mn2Al specimen.

Effect of superplastic forming exposure on fatigue crack propagation behavior of Ti-6Al-4V alloy

The effect of superplastic forming (SPF) exposure on the ε (strain)-N (number of cycles to failure) fatigue and fatigue crack propagation (FCP) behaviors of Ti-6Al-4V (Ti64) alloy was examined at 298 and 473 K. To simulate the thermal exposure during superplastic forming process, the mill-annealed Ti64 alloy sheet was heated in the vacuum chamber with the pre-determined temperature profile. Notable microstructural change during the SPF exposure included the shape of transformed β phase from fine and round particles in the as-received specimen to coarse angular particles in the as-exposed specimen. The effective grain size tended to increase with the exposure, enhancing the slip reversibility and the resistance to FCP. However, the crack hindering effect by fine, particle-like β phase became weak with the exposure, offseting the beneficial effect associated with the increment of effective grain size. The effect of SPF exposure on ε-N fatigue and FCP behavior of mill-annealed Ti64 alloy was therefore marginal, excluding the effect of α-case (the oxygen-enriched phase) on the surface.